Nitrogen fixation in the widely distributed marine γ-proteobacterial diazotroph <i>Candidatus</i> Thalassolituus haligoni.

Abstract

The high diversity and global distribution of heterotrophic bacterial diazotrophs (HBDs) in the ocean has recently become apparent. However, understanding the role these largely uncultured microorganisms play in marine N₂ fixation poses a challenge due to their undefined growth requirements and the complex regulation of the nitrogenase enzyme. We isolated and characterized <i>Candidatus</i> Thalassolituus haligoni, a member of a widely distributed clade of HBD belonging to the Oceanospirillales. Analysis of its <i>nifH</i> gene via amplicon sequencing revealed the extensive distribution of <i>Cand.</i> T. haligoni across the Pacific, Atlantic, and Arctic Oceans. Pangenome analysis indicates that the isolate shares >99% identity with an uncultured metagenome-assembled genome called Arc-Gamma-03, recently recovered from the Arctic Ocean. Through combined genomic, proteomic, and physiological approaches, we confirmed that the isolate fixes N₂ gas. However, the mechanisms governing nitrogenase regulation in <i>Cand.</i> T. haligoni remain unclear. We propose <i>Cand.</i> T. haligoni as a globally distributed, cultured HBD model species within this understudied clade of Oceanospirillales.